When to use a standalone RTC IC instead of an MCU embedded RTC in low power IoT devices

When Real Time Clock ICs (RTC ICs) appeared in the semiconductor market around the late nineties, their primary purpose was to keep the time and provide data and time information in a more usable way compared to a simple clock counter. Since then, these products have evolved, introducing several new features such as alarms, watchdog, timestamp recording, embedded memory, and various others. They also reduced the power consumption to shallow values. However, in the last ten years, the RTC function has often become an integrated function in MCUs (MCU-RTC) with performances like standalone RTC ICs.

So why use standalone RTC ICs? Moreover, if so, for which applications and to what extent?

We’ll look at standalone RTCs used in some specific applications, such as IoT, where either the power and the accuracy are key factors. A one-to-one comparison on features and electrical parameters can help designers to take the most value from a standalone RTC IC.

Key requirements in IoT and other low power applications

IoT applications embrace several types of devices: from powerful multi-core smartphones to small connected sensors. However, extended battery lifetime is a common need for all these devices. In particular, this requirement is fundamental in the IoT devices with small batteries or those powered by harvested energy. Figure 1 shows the typical block schematic of a low power IoT device. The MCU manages the various functions of the device. It decides to turn-on or -off the peripherals according to the application conditions. The on/off time management of the various functions is a key way to reduce power consumption. It is minimized if the MCU goes in deep sleep mode when not needed and if the time base is accurate enough not to create significant time shift between the wake-up signal and the actual need of the peripheral activation.

Figure 1: Typical IoT Architecture

Most RTC ICs have a very flexible interrupt management that allows waking-up the host MCU from millisecond periods to yearly periods. Even if the MCU in RTC mode has the same function, the RTC ICs interrupt management is entirely independent from the software execution, which allows full sleep mode of the MCU and improves the robustness of the application.

When compared to the MCU current draw in RTC mode—in the range of a few micro amperes—RTC ICs provide these precise interrupts with current consumption below a few hundred nano amperes—a significant current reduction.

Timekeeping and backup power

For all IoT devices, keeping time is crucial. In the absence of the main power source, having reliable information of event occurrences and communication time slots is accomplished with the use of either a small battery or a capacitor.